Work done in animals in our laboratory showed a total inhibition of analgesic effects of paracetamol administered orally, intravenously or intrathecally after intrathecal administration of tropisetron, a 5 HT3 receptor antagonist. This suggests a role of 5-HT3 receptor in the mechanism of action of paracetamol, despite the absence of binding of acetaminophen to this receptor.